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United States Patent |
5,116,460
|
Bukhman
|
May 26, 1992
|
Method for selectively etching a feature
Abstract
A method is provided for selectively etching materials on a semiconductor
wafer (10, 30) that have similar etch rates. The semiconductor wafer (10,
30) is provided with at least a first layer. An etch mask is provided on
the first layer. The layer with the etch mask (13) is partially etched to
a predetermined point. A polymer film (21, 38) is deposited on the
partially etched layer. The polymer film (21, 38) is etched in an
anisotropic manner creating open or clear areas (14, 34) in the horizontal
polymer film, while leaving polymer coating (22, 37) on vertical walls
(12, 36). The open areas (14, 34) are chemically etched, while the
remaining polymer coating (22, 37) on the vertical walls (12, 36) protects
the vertical walls (12, 36) from being chemically etched. This method also
protects the top surface of the semiconductor wafer.
Inventors:
|
Bukhman; Yefim (Scottsdale, AZ)
|
Assignee:
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Motorola, Inc. (Schaumburg, IL)
|
Appl. No.:
|
684130 |
Filed:
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April 12, 1991 |
Current U.S. Class: |
438/696; 257/E21.235; 257/E21.257; 257/E21.259; 257/E21.314 |
Intern'l Class: |
B44C 001/22; B29C 037/00 |
Field of Search: |
156/643,646,649,651,652,653,655,657,659.1,662,668
204/192.32,192.36,192.37
437/228,235,238,241
|
References Cited
U.S. Patent Documents
4604162 | Aug., 1986 | Sobczak | 156/649.
|
4657630 | Apr., 1987 | Agatsuma | 156/643.
|
4661374 | Apr., 1987 | Doering | 156/652.
|
4805683 | Feb., 1989 | Magdo et al. | 156/643.
|
4818714 | Apr., 1989 | Haskell | 156/652.
|
4838991 | Jun., 1989 | Cote et al. | 156/643.
|
Primary Examiner: Powell; William A.
Attorney, Agent or Firm: Barbee; Joe E.
Claims
I claim:
1. A method for selectively etching a feature in layers having similar
anisotropic etch characteristics on a semiconductor wafer comprising:
providing a semiconductor wafer with a first and a second layer;
providing an etch mask on the second layer, which defines the second layer
into open areas and covered areas that are protected by the etch mask;
etching the open areas of the second layer partially until a predetermined
amount of the open areas is removed;
depositing a polymer film over the partially etched open areas and the
covered areas;
anisotropic etching of the polymer film, leaving walls of the etched open
areas protected by the polymer film while the other areas are etched clear
of polymer; and
chemically etching the open areas that are clear of polymer.
2. The method of claim 1 further comprising using a dry chemical means for
chemically etching the open areas that are clear of the polymer film.
3. The method of claim 1 further comprising using a wet chemical means for
chemically etching the open areas that are clear of the polymer film.
4. The method of claim 1 further comprising depositing the polymer film in
situ.
5. The method of claim 1 wherein etching of the second layer, depositing of
the polymer film, anisotropic etching of the polymer film, and chemically
etching of the open areas is accomplished in one system.
6. A method for selectively etching a first layer over a semiconductor
substrate comprising:
providing the semiconductor substrate with a first layer that has at least
one etch mask defined on the first layer;
etching the first layer in an anisotropic manner until a predetermined
depth in the first layer is reached, thereby producing at least one wall;
covering the etch mask, the first layer that has been partially etched to
the predetermined depth, and the at least one wall with polymer film;
anisotropic etching of the polymer film so that areas are formed that are
cleared of the polymer film, exposing the partially etched first layer and
leaving the polymer film on the at least one sidewall; and
etching the first layer that has been partially etched with an isotropic
chemistry that is selective to the semiconductor substrate.
7. The method of claim 6 further comprising having the semiconductor
substrate be silicon and having the first layer be silicon dioxide.
8. The method of claim 6 further comprising using photoresist for the etch
mask.
9. The method of claim 6 further comprising using either a wet or dry
isotropic chemistry for etching the first layer that has been partially
etched.
10. The method of claim 6 wherein etching of the first layer, covering with
the polymer film, anisotropic etching of the polymer film, and chemical
etching of the clear areas is accomplished in one system.
11. The method of claim 6 further comprising removal of remaining polymer
film by a wet or dry means.
12. A method for making a sidewall spacer on a semiconductor wafer
comprising:
providing a feature with at least one sidewall on a semiconductor wafer;
providing a first conformal layer on the feature and on the sidewall;
etching the first conformal layer in an anisotropic manner until a
predetermined amount of the first layer is removed;
depositing a polymer film on the first etched layer and on the sidewall;
etching the polymer film in an anisotropic manner, thereby removing
portions of the polymer film, which creates open areas and leaves the
polymer film on the sidewall of the etched first layer; and
etching open areas of exposed etched first layer by chemical means until
the exposed first layer is completely etched away and leaving the polymer
coated sidewall.
13. A method of selectively etching a semiconductor wafer comprising:
providing a feature with at least one sidewall on a semiconductor wafer;
providing a first conformal layer that covers the feature and the sidewall
forming a second sidewall;
providing a second conformal layer that is deposited on the first layer;
etching the second layer in anisotropic manner until a pre-determined
amount of the second layer is removed;
depositing a polymer coating on the second etched layer;
etching in an anisotropic manner the polymer coating, thereby removing the
polymer coating in open areas and leaving the polymer coating on the
sidewall; and
etching the second layer until completion and leaving the polymer coated
sidewalls.
14. The method of claim 13 wherein the first and second conformal layers
are a conformal oxide layer and a conformal nitride layer respectively and
that are adjusted to a thickness between 50 angstroms to 500 angstroms and
to between 1,500 angstroms to 3,000 angstroms respectively.
15. The method of claim 14 further comprising providing a conformal oxide
layer that is substantially adjusted to a thickness of 100 angstroms
16. The method of claim 14 further comprising providing a conformal nitride
layer that is substantially adjusted to a thickness of 3,000 angstroms.
17. The method of claim 13 wherein the polymer coating has a thickness
between 500 angstroms to 1,000 angstroms.
18. The method of claim 17 wherein the polymer coating is substantially
adjusted to a thickness of 750 angstroms.
19. A method of selectively etching at least a first layer on a substrate
comprising:
etching selective areas of the first layer with a high energy plasma etch
to form at least one side wall;
stopping short of etching through the first layer;
covering the first layer and the at least one side wall with a polymer
film;
anisotropically etching all horizontal surfaces of the polymer film to
leave the polymer film of the at least one sidewall;
isotropically etching the remainder of the first layer not covered by the
polymer film; and
removing the polymer film, thereby exposing the at least one sidewall.
Description
BACKGROUND OF THE INVENTION
This invention relates, in general, to manufacturing semiconductor
products, and more particularly to etching features used in semiconductor
devices for semiconductor products.
Generally, etching is a pattern transfer process that has been used in
manufacturing semiconductor devices for a long time. Basically, the
process requires that a masking layer be defined with a pattern. The
masking layer is placed over or on top of a film that the pattern is to be
transferred into. The film is then removed or etched away from around the
masking layer, leaving an identical pattern that was previously defined by
the masking layer in the etched film. Additionally, some features can also
be fabricated by etching whole films without the use of the masking layer.
A large portion of etching of semiconductor devices is achieved
conventionally by using gaseous plasma processes. These plasma processes
are generally known as plasma etching. By selecting appropriate process
conditions, the gaseous plasma can be made to be a predominantly chemical
process, a predominantly physical process or a combination of both
chemical and physical processes. Selecting either a predominantly chemical
process or a predominantly physical process, results in different
structural effects in the etched film. Chemical processes etch in an
isotropic manner and do not exhibit dimensional control, whereas physical
processes etch in an anisotropic manner and do exhibit dimensional
control. Further, chemical or isotropic etching processes generally do not
damage an underlying layer or a substrate which is beneath the layer that
is being etched; however, anisotropic etching processes typically use high
potential plasmas which do damage the underlying layer or substrate
beneath the layer that is being etched.
Additionally, by adjusting the process conditions of the gaseous plasma,
process parameters, such as etch rate and selectivity, can be adjusted and
changed. Etch rate or removal rate is a parameter that indicates a speed
at which a material is being removed. Selectivity is the etch rate or the
removal rate of two or more materials that are compared to each other for
a given set of plasma conditions. Until recently, adjustment of the
gaseous plasma to either a predominantly chemical process or a
predominantly physical process was sufficient to obtain desired results in
regards to structural effects, etch rates, and selectivities.
However, as semiconductor products have become more complicated and etch
requirements have become more stringent, several problems have occurred
with achieving the desired selectivities, while maintaining desired
structural effects. One problem occurs when a highly selective etch is
required between two materials that etch at similar etch rates, and are
etched simultaneously. By using conventional adjustment methods, it is not
possible to etch these materials with high selectivity and with
dimensional control. Not being able to achieve these requirements, makes
it impossible to manufacture some structures or degrades the semiconductor
device that are manufactured.
Additionally, problems of dimensional control and damage to an underlying
layer or substrate are exacerbated when plasma etching of features with
high aspect ratios, such as when fabricating sidewall spacers. By using
conventional methods, it is not possible to etch features with high aspect
ratios and still maintain dimensional control, high selectivity, and low
damage to underlying structures.
Therefore, a method to achieve highly selective etches and to have a
greater dimensional control would be very desirable. Additionally, having
a method that protects the semiconductor device from radiation damage or
etch damage from energetic ions would also be desirable.
SUMMARY OF THE INVENTION
Briefly, according to the invention, a method is provided for etching
materials selectively that have similar etch rates on a semiconductor
wafer. The semiconductor wafer is provided with a first and a second
layer. An etch mask is provided on the second layer, which is partially
etched to a predetermined point. A polymer film is deposited on the
partially etched second layer. The polymer film is etched in an
anisotropic manner creating open or clear areas in the polymer film, while
leaving some of the polymer film on walls of the partially etched second
layer. The open areas are chemically etched, while the remaining polymer
on the vertical walls protects the vertical walls from being chemically
etched.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. lA and FIG. IB illustrate a cross-sectional view of a portion of a
semiconductor device in various stages of practicing the present
invention; and
FIG. 2A and FIG. 2B illustrate a cross-sectional view of a portion of a
semiconductor device in various states of practicing the present invention
in another embodiment.
DETAILED DESCRIPTION OF THE DRAWINGS
FIG lA is a cross-sectional view of a portion of a semiconductor substrate
10 with a partially etched feature 11 that is covered by a polymer coating
21. It should be understood that only a small portion of semiconductor 10
is shown and that many more features, such as feature 11, can be on
semiconductor substrate 10. It should be further understood that feature
11 was formed from a first layer on semiconductor substrate 10; however,
substrate 10 could just as well have been a second layer of material. For
illustrative purposes only, use of semiconductor substrate 10 was chosen.
A first layer of material is either deposited or grown on semiconductor
substrate 10 by using methods known in the art. The first layer can be
made of many different kinds of materials, such as dielectrics,
conductors, III-V semiconductor materials, or other semiconductor
materials. In a preferred embodiment, the first layer is a dielectric,
such as silicon dioxide. The first layer is not shown in FIG. IA as a
continuous layer, however the first layer is shown as a partially etched
feature 11 with thin areas 16. Conventionally, an anisotropic etch is used
to maintain dimensional control or transfer the width of the pattern into
the layer or the substrate that is being etched. Typically, in order to
achieve this transfer, a high energy plasma that etches in a vertical
direction is used. The first layer is patterned by well-known methods in
the art.
In the present invention, the first layer is only partially etched forming
feature 11 on semiconductor substrate 10. Feature 11 is etched by an
anisotropic plasma etching process, thereby, yielding vertical or near
vertical sidewalls 12 with a width that is similar or the same as the
width of etch mask 13. Additionally, by anisotropic or directional etching
very little or no damage is done to sidewalls 12, because of the vertical
nature of anisotropic etching. It is also important to realize that the
first layer is only partially etched to completion and that a thin layer
16 still remains. By not etching thin layer 16 to completion, surface 19
is protected from ion bombardment and radiation damage which normally
occurs with conventional anisotropic plasma etching methods that would
etch the first layer to completion. The ion bombardment and radiation
damage that some substrates, such as silicon, sustain during conventional
etching using energetic ions makes these exposed substrates unsuitable or
marginal for building devices on. Typically, in the present invention, the
first layer is etched to a predetermined depth that is approximately 85
percent to 95 percent completion; however, the percent completion or
desired stopping point is dependent upon uniformity of the anisotropic
etching process. Knowing when to stop etching of the first layer can be
achieved by many methods, such as time, laser interferometry, or other
end-point detection methods.
After partial etching of feature 11 is completed, a conformal polymer
coating 21 is deposited onto partially etched feature 11 and all
associated exposed surfaces on semiconductor substrate 10. Generally, this
deposition is achieved by changing plasma chemistry from an etching mode
to a conformal polymer deposition mode. However, it should be realized
that even though it is preferred to deposit polymer coating 21 in the same
reactor that was used to etch feature 11, it is also possible to achieve
deposition of polymer coating 21 in another reactor. There is a variety of
reaction chemicals and plasma conditions that can create polymer coating
21. Typically, these chemicals are fluorocarbons that are reacted at
relatively high pressures and low powers. Solely, for illustrative
purposes, examples used in FIG. lA will be limited to having semiconductor
substrate 10 being made of silicon, and feature 11, that was fabricated
from the first layer, being made of silicon dioxide. It should be
understood that other substrates and other layers and features could also
be used.
By way of example, polymer coating 21 is deposited on all exposed surfaces
of semiconductor substrate 10 to an approximate thickness between 500
angstroms to 1,500 angstroms. This deposition typically is accomplished
with a plasma made of fluorocarbon gases, such as CHF.sub.3, C.sub.2
H.sub.2 F.sub.2, or the like. Plasma conditions typically are between 250
watts to 1,500 watts, with chamber pressures that are between 100
millitorr to 1.5 torr.
FIG. lB is a cross-sectional view of a portion of semiconductor substrate
10, with partially etched feature 11 that has had horizontal portions of
polymer coating 21 etched away by an anisotropic etch. By etching
conformal polymer coating 21 in an anisotropic manner all horizontal
surfaces that were covered by conformal polymer 21 are etched away, thus
exposing surfaces 14 and 17. By way of example, polymer coating 21 is
etched typically in an anisotropic manner in a plasma that oxidizes
conformal polymer coating 21 into volatile gases. Oxidation plasma
chemistries can be created by gases, such as oxygen. Generally,
anisotropic conditions are achieved by using low pressures and high
powers. Typical ranges of pressure are between 1.0 millitorr to 200
millitorr, with powers ranging between 500 watts to 1,500 watts. Etching
away exposed horizontal surfaces of conformal polymer 21, shown in FIG.
IA, results in exposing silicon dioxide surfaces 14 and photoresist
surface 17, while protecting silicon dioxide sidewalls 12 with polymer
coating 22.
Additionally, sidewalls 12 are still covered by a polymer coating 22 that
was part of conformal polymer coating 21. By having sidewalls 12 covered
by polymer coating 22, protection is given to sidewalls 12. Exposed
horizontal surfaces 14 are now capable of being chemically etched without
affecting sidewalls 12 and damaging surface 19. Typically, etching of
surface 14 is achieved by an isotropic or a chemical means; therefore,
etching of thin layer 16 is achieved vertically and horizontally at equal
rates. However, since sidewalls 12 are protected by polymer coating 22 and
the thickness of thin layer 16 is small, complete etching or removal of
thin layer 16 affects feature 11 only slightly or not at all.
Additionally, by chemically etching away thin layer 16 damage to
semiconductor substrate 10 itself is prevented. Therefore, removal or
etching away of thin layer 16 can be accomplished without damaging
sidewalls 12 and without damaging substrate 10.
Isotropic or chemical etching can be achieved either by dry plasma etching
or by wet chemical etching. In this particular case, a wet chemical etch
of dilute aqueous hydrofluoric acid would be preferred for removing the
thin oxide layer 16. By etching thin oxide layer 16 away with dilute
aqueous hydrofluoric acid, a semiconductor device quality silicon
substrate 19 is exposed. Since silicon substrate 19 has not been directly
exposed to any ion bombardment or radiation from the anisotropic gaseous
plasma, damage that would have been caused by such does not occur in the
present invention.
Once removal of thin layer 16 is accomplished, normal conventional methods
and techniques can be used to remove remaining polymer 22 in an isotropic
or chemical manner, such as a hydrogen peroxide and sulfuric acid
solution, or an oxygen plasma.
FIG. 2A is a cross-sectional view of a portion of a semiconductor substrate
30 with several structural layers 32 and 34 on an etched feature 31. Layer
34 and sidewalls 36 originated from a thick conformal layer that was
etched. Layers 32 and 34 are used to fabricate sidewalls 36 around feature
31. Etched feature 31 can be fabricated by many methods known in the
semiconductor art. Etch feature 31 can be made of many different kinds of
materials used in fabricating semiconductor devices, such as silicon,
polysilicon, III-V semiconductor materials, metals, and metal alloys. Etch
feature 31 is covered by a conformal thin layer 32, which is either
deposited or grown over etched feature 31. Thin layer 32 can also be made
of several materials, such as oxide or nitride. Subsequently, a thick
conformal film is deposited on conformal thin layer 32. Additionally, the
thick conformal layer can also be made of several materials, such as oxide
or nitride. Both thin conformal layer 32 and the thick conformal film are
made by using known methods in the art.
In the present invention, the thick conformal film is etched to a
predetermined depth that is approximately 85 to 95 percent completion;
however, the percent completion or desired stopping point is dependent
upon uniformity of the anisotropic etching process. This etch is achieved
in a uniform vertical manner without a mask. By etching the thick
conformal film by an anisotropic manner, horizontal areas 34 are thinned,
while vertical or sidewalls 36 remain approximately the same thickness as
the thick conformal layer before etching. Knowing when to stop the thick
nitride etch can be achieved by many methods, such as time, laser
interferometry, or end-point detection. After etching the original thick
conformal film in an anisotropic manner, a polymer coating 38 is deposited
in a conformal manner over the entire etched surface.
Polymer coating 38 is deposited in a plasma reactor in the same manner as
polymer coating 21 discussed previously in FIG. lA.
For the sake of simplicity, examples used to illustrate the present
invention will be of a single structure. The use of the single structure
by no means is intended to limit the many possible materials in the
present invention. The single structure will have feature 31 being made of
silicon, thin conformal layer 32 being made of oxide, and a thick
conformal layer (now thin layer 34 and sidewall 36 being made of silicon
nitride. Typically, thin oxide layer 32 and thick nitride film have
thickness ranges between 50 angstroms to 300 angstroms and 3,000 angstroms
and 5,000 angstroms respectfully.
FIG. 2B is a cross-sectional view of a portion of semiconductor substrate
30, with partially etched feature 11 that has had the horizontal portions
of polymer coating 38 etched away by an anisotropic means. By having
polymer 38 etched in an anisotropic manner, horizontal surfaces are etched
cleanly and are free of polymer 38, while leaving a portion of polymer 38,
shown as polymer 37 to protect sidewalls 36. This protection allows for
further processing to remove thin material areas 34, which are now capable
of being etched away by using chemical or isotropic etching methods.
Chemical or isotropic etches can be either dry plasma etches or wet
chemical etches depending upon what materials are to be removed. Using a
low energy gaseous plasma, a predominantly chemical plasma, either avoids
or abates damage to the underlying layers by not having high energy ions
bombarding thin materials 34 and eventually layer 32. Therefore, use of
chemical or isotropic etches, which include both wet and gaseous plasmas
can be selected depending upon materials that are to be etched away.
Further, by having protected the sidewalls 36 by polymer 37, thin material
areas 34 can now be removed without damaging thin layer 32 and without
loss of dimensional control of sidewalls 36. Conventionally, in order to
etch the thick film without damaging thin layer 32 and without loss of
dimensional control of sidewalls 36, process conditions would be adjusted
to the very best selectivity; however, if sidewalls 36, thin material area
34, and thin film 32 all etch at similar rates, adjustment of the process
conditions cannot cause the desired effect of increasing the selectivity.
Additionally, protecting sidewalls 36 with polymer coating 37 allows for
etching of thin material 34 in an isotropic manner without affecting
sidewall 36 dimensional control.
Typically, removal of the horizontal areas of polymer 38 is achieved by
exposing the horizontal areas of polymer 38 to an oxidizing plasma with an
anisotropic nature. This plasma is achieved by using oxidizing gases, such
as oxygen. Typical process conditions for such a plasma would have
pressures between 1.0 millitorr to 200 millitorr and power ranges between
500 watts to 1,500 watts.
By having the anisotropic oxidizing plasma remove horizontal polymer area
38, exposes thin silicon nitride areas 34 that are ready for removal by
chemical or isotropic etching methods. Using chemical or isotropic etching
methods to remove thin silicon nitride areas 34 ensures that removal is
achieved without damaging oxide layer 32. Additionally, polymer 37
protects sidewalls 36 while isotropic etching of thin layer 32. In this
particular example, it is preferred that a dry chemical or isotropic
plasma etch be used to remove thin silicon nitride areas 34. The dry
chemical plasma typically uses a fluorocarbon plus an oxygen gas mixture
in a reactor. The fluorocarbon reacts with thin silicon nitride to form
volatile gas species, while the oxygen reacts and removes polymer that is
deposited by the fluorocarbon. Even though some of polymer 37 may be
etched by the gas mixture, it should be understood that the thickness of
polymer 37 that was deposited previous to etching of silicon nitride areas
34, is thick enough to withstand this slight etching while thin silicon
nitride areas 34 are removed. Additionally, in this particular example,
chemical or isotropic etch rates of silicon nitride and oxide are
considerably different, with silicon nitride etching much faster than
silicon dioxide. However, anisotropic etch rates of silicon nitride and
silicon dioxide are quite similar. Therefore, using a chemical or
isotropic etching method to remove thin nitride areas 34, results in a
selective etch and that does not expose oxide layer 32 to a high energy
anisotropic plasma. By etching with an isotropic plasma it should be
understood that etching occurs in all directions at the same rate.
Additionally, by using an isotropic etching means, damage to thin layer 32
is negligible. Therefore, since thin silicon nitride layer 32 is between
250 angstroms to 750 angstroms in thickness, only a very small amount, if
any, is going to be removed from under sidewall 36.
After etching of horizontal surface 34 is complete, removal of remaining
polymer 37 is achieved by normal conventional methods, such as wet
chemical stripping with hydrogen peroxide and sulfuric acid, or an oxygen
plasma.
In FIG. lA and FIG. IB a method is described that allows for etching of
feature 11 while maintaining control of width dimensions and not damaging
fragile surface 19. FIG. 2A and FIG. 2B a similar method is described
except that multiple layers have been either deposited or grown over an
already existing feature 31, so that sidewalls 36 can be fabricated.
By now it should be appreciated that there has been provided a method for
selectively etching features into materials with similar etch rates
without damaging the underlying surface or substrate, as well as
controlling critical dimensions. Additionally, a method is provided for
etching sidewall spacers of high aspect ratios with high selectivity with
materials that ordinarily have low selectivity.
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